This invention relates generally to doubleface corrugated paper board, and more particularly to a dryer-cooling machine adapted to combine a singleface web with a liner web and to cure the adhesive which bonds the webs together to form doubleface corrugated board.
Singleface corrugated paper board consists of a fluting ply whose troughs are laminated to a base liner. To fabricate doubleface board, a second liner must be bonded to the peaks of the fluting of the singleface board.
In a typical combining machine now in use for this purpose, the singleface web, after going through an applicator which applies wet adhesive to the peaks of the fluting, is conducted through the machine concurrently with a liner web. The combining machine includes an elongated table provided with a heater section and a cooling section. The liner web is drawn along the surface of the table with the singleface web overlying the liner web so that the flute peaks are in contact with the liner. In the heater section, the table functions as a hot plate and in the cooling section as a cold plate.
An endless cotton belt supported above the table and operating in conjunction with a row of pressure rolls serves to press the flute peaks of singleface web tightly against the liner web as these webs travel along the table to produce intimate paper contact with proper adhesive wetting. Pressure in the regions between the rolls must be sufficient to maintain paper contact during the initial stages of adhesive curing; for if the paper surfaces are permitted to separate, the fragile bond formations will be fractured and the glue line will cure with excessive bridging.
The adhesive normally used in producing doubleface board is a starch/water slurry. This slurry is transformed into a gelatinized adhesive at about 150.degree. F. and finally into a cured adhesive after most of the water has been extracted by evaporative dehydration at 212.degree. F. Polymer molecules form the strong crystallized structure which is characteristic of the cured adhesive.
Tests indicate that about 80 BTU's per pound of liquid adhesive are required to gelatinize the starch granules, while an additional 644 BTU's are necessary to evaporate 80% of the water. Typically, this corresponds to 2.94 BTU's to cure the doubleface glue lines in one square foot of combined board. In the conventional combining machine of the table type, this energy must be transmitted from the hot plate section through the lower liner web to reach the glue lines. As heat energy flows through this web, it heats and dries the paper fibers. Since the absorption by the paper fibers removes water from the glue line at a rather slow rate, rapid removal by evaporative boiling is essential to obtain the required bond strength at production speeds.
In combining machines of the type heretofore known, in order to produce doubleface corrugated board of good quality, the table must be long in order to afford adequate heating and cooling time, and the rate of web travel must be relatively slow in order to avoid dimensional distortion of the product. Because the lower liner web slides along the stationary table in frictional contact therewith, and the singleface web is forced thereagainst by the belt and pressure rolls which overlie this web, the webs are squeezed between the moving belt and the stationary table.
This arrangement dictates a slow speed operation; for should one attempt to pull the squeezed webs through the machine at an increased rate, this may result in extrusion and distort the product. Hence, existing combining machines for producing doubleface corrugated board of good quality are slow and inefficient in terms of productivity.
Another drawback of existing combining machines resides in the fact that should the moving webs being combined be brought to a halt on the table because of some fault in the drive mechanism or for some other reason, the heat from the heater section will then scorch the webs and render them unacceptable.